The present invention relates to a machine tool. More particularly, the present invention relates to an ultrasonic machine that has, on one end of a main shaft, a machine head provided with a piezoelectric transducer for exciting a tool attached to the machine head in ultrasonic vibrations. The present invention also relates to a machining method in which a tool head with grindstone on the tip thereof is rotated and ultrasonically vibrated at the same time to machine the work.
Conventionally, machine tools that have, on a main shaft, a machining head provided with a piezoelectric transducer are used to ultrasonically machine hard materials such as ceramics and glass. Japan Published Unexamined
Utility Model Application No. 64-56941 discloses one such machine tool comprising a machine head which is provided with a piezoelectric transducer and a housing thereof. The housing is mounted on the machining head via a large-diameter bearing that encircles the machining head. Slip rings are provided on the periphery of the machining head in order to connect the piezoelectric transducer of the rotatable machining head to a power source. On the other hand, brushes are provided on the inner surface of the housing that are in slidable contact with the slip rings. The brushes are connected with the power source via cables.
However, in such a conventional machine tool, a machining method suitable for the tool on the work sometimes cannot be adopted because of the difficulty of rotating the machining head at a high speed. This is because the large-diameter bearing provided between the machining head and the housing is vulnerable to high speed rotation. The vulnerability cannot be overcome by providing the large-diameter bearing with various lubricating means as is the case with the main shaft because the machining head is small in size. Furthermore, the brushes being in touch with the slip rings at all times also hampers high-speed rotation. If the tool is no more than 1 mm in diameter, the slow rotation does not allow the periphery of the tool to travel fast enough. This causes various problems including a low machining efficiency and a short life of a tool.
A known method of ultrasonically machining hard, brittle materials such as ceramic and glass uses an ultrasonic machine equipped with a grindstone such as diamond for boring or polishing. More particularly, the main shaft is rotated by a motor while the piezoelectric transducer is ultrasonically vibrated at the same time. Then, the top of the tool is pressed onto the work. This causes the tool to vibrate in the direction toward the work and to rotate at the same time on the surface of the work, thereby boring into the work.
However, in the above conventional method of ultrasonic machining, the rigidity of the tool becomes too low if the diameter thereof is very small, for example no more than 1 mm. At an initial stage of the machining when there is no guide hole yet made, the tip of the tool tends to bend and slide, making an eccentric rotation on the surface of the work without making a bore. Moreover, the above problem may also cause the grindstone to break away from the tool.
In the conventional method, therefore, the smaller the diameter of the tool is, the more difficult boring is and the shorter the life of a tool is.
Wherefore, an object of the present invention made to overcome the above-identified problems is to provide an ultrasonic machine that can rotate the machining head at a high speed.
Another object of the present invention made to overcome the above-identified problems is to provide an ultrasonic machining method in which boring of the work is effectively carried out and the life of the tool is prolonged.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.